A digital communication system receiver typically incorporates a clock and data recovery (CDR) system. Such a CDR system may be configured to process a received serial data stream in order to generate a clock signal that is used for data sampling. The CDR system usually includes a phase control loop that attempts to lock the clock signal frequency and phase to the frequency and phase of the received serial data stream. After a lock condition has been achieved, the phase control loop continues to adjust the clock signal frequency and phase so as to track the frequency and phase of the serial data stream. In the event of a loss of lock, the phase control loop has to re-lock to the serial data stream.
Accordingly, it is important to provide an accurate loss of lock detector for a CDR system. This allows the CDR system to efficiently initiate re-locking to the serial data stream in the event of a loss of lock. The loss of lock detector may provide a binary indicator signal having a first logic level indicating that the clock signal is properly locked to the serial data stream, and a second logic level indicating that the clock signal is not properly locked to the serial data stream.
Exemplary loss of lock detection techniques are disclosed in U.S. Pat. No. 8,208,521, entitled “Methods and Apparatus for Detecting a Loss of Lock Condition in a Clock and Data Recovery System,” which is commonly assigned herewith and incorporated by reference herein. In one technique disclosed therein, a loss of lock condition is detected in a CDR system by sampling the received serial data stream for different phases using one or more latches clocked by the recovered clock signal, evaluating the samples to monitor a data eye associated with the serial data stream, and detecting the loss of lock condition if the data eye does not satisfy one or more predefined conditions. The corresponding loss of lock detector is also referred to herein as a rotational detector.
By way of example, the one or more predefined conditions can be selected to ensure that the additional data samples generated for the different phases have a deterministic pattern when the CDR system is in a lock condition. As a result, loss of lock can be detected by determining if the additional data samples have a non-deterministic pattern.
Despite the considerable advantages provided by the techniques disclosed in the above-cited U.S. patent, a need remains for further improvements in loss of lock detection for CDR systems.